The Siege of Byzantium was carried out sometime between July and September 324 by the forces of the Roman emperor Constantine I (r. 306–337) during his Second Civil War against his rival, co-emperor Licinius (r. 308–324). It would have been started simultaneously with the naval battle of the Hellespont (today known as Dardanelles) in which Constantine's son and caesar Crispus (r. 317–326) defeated the Lycinian navy commanded by Admiral Abanto.